1. Field of the Invention
This invention relates to austenitic stainless steels which are superior in hardness and corrosion resistance and employed in such plants as chemical, sea water and nuclear plants.
2. Description of the Prior Art
Austenitic stainless steels are widely employed in the construction of plants because of their superiority in corrosion resistance, heat resistance, workability, and mechanical properties. Typical austenitic stainless steels are such steels as SUS304, SUS316, SUS304L, SUS316L. As machines and building structures have become larger recently, the improvement in the strength of structural stainless steels has been desired. And, the steels, which are made of the above steels with addition of nitrogen and niobium, such as SUS304N.sub.1, SUS304N.sub.2, SUS304LN, SUS316N, and SUS316LN are well-known.
The strength of these steels after solution heat treatment, however, is not sufficient although the yield strength of SUS304LN and SUS316LN equals to or exceeds 25 kg/m.sup.2 and SUS304N.sub.1 and SUS316N possess yield strengths exceeding 28 kg/mm.sup.2. The yield strength of SUS304N exceeds 35 kg/mm.sup.2, which is a relatively high strength, but not always sufficient.
Various techniques have been studied in recent years to further improve the strength of these steels. And, cold-working and control rolling, for example, have been suggested for the purpose.
Cold-working, however, requires a reduction ratio exceeding 20% to produce a desired strength. For this reason, cold-working is employed only for sheet steels and wire rods.
Control rolling cannot provide sufficient strength for SUS316LN, 316N, 316L, 316, 304LN, 304N.sub.1, 304 and 304L, and causes SUS304N.sub.2 to crack during hot-working. And this technique cannot provide sufficient corrosion resistance for SUS304N.sub.2, either. Thus, control rolling has not been applied in practical use.
Further SUS304N.sub.1, SUS304N.sub.2 and SUS316N are inferior in intergranular corrosion resistance and stress corrosion cracking susceptibility resistance, and the corrosion resistance of these steels decreases sharply after welding.